[llvm] [Offload][L0][NFC] Remove unused zexKernelGetArgumentSize dispatcher (PR #227589)

[Alex Duran via llvm-commits]

adurang wrote:

#228070 is making use of zexKernelGetArgumentSize again, so maybe it's best not to remove here? @blazej-smorawski
?

https://github.com/llvm/llvm-project/pull/227589